um, wtf? do you wanna **** her?
well theres your answer...
If you feel right about doing it, go for it. Just because you are not a lesbian, and I hate definitions, and the possibility that your girlfriend is also not a lesbian, having sex with her doesn't define either of you as being lesbians.
When I was in university, many years ago, lots of the girls carried the title of 'L.U.G.', meaning Lesbian Until Graduation. Lot's of them talked about having sex with their roommates, but they wouldn't have sex with a man. I still know some of them today and they are as straight as can be.
Its fine talk to her listen to u and only u
If you want to -- if you don't --- don't
If thats what you want and you are not doing it just to make her happy
try it. you might like it